This invention relates to the removal of particulate material from gas streams. More specifically, this invention relates to the removal of particulates having a particle size in the micron and submicron range from relatively hot gas streams.
Particulate laden gas streams are produced by a large variety of industrial processes. Depending upon the source, temperature of such gas streams can range from ambient to combustion temperatures. Exhaust gases from combustion processes represent probably the most common examples of such particulate carrying gas streams. Other troublesome gas streams include emissions from cupola furnaces, exhaust streams from cement kilns, coke oven off-gas and quench gas, exhaust gases from coffee roasting and grain drying and other similar processes.
In addition to carrying particulate matter, most of these waste gas streams are also at moderately elevated to very high temperatures. It is standard to remove particulates from such gases by a variety of techiques including electrostatic precipitation, filtration such as in bag houses, and by the use of a variety of wet-scrubbing techniques. Wet scrubbers generally find use where cooling of the gas stream is desired, where moisture addition is not objectionable and where the problem of disposing or further treating the scrubbing liquid polluted with the materials removed from the gas can be handled. Publications describing the uses, design and performance of wet scrubbers are:
Calvert, Seymour, How to Choose a Particulate Scrubber, CHEMICAL ENGINEERING, pp. 54-68, Aug. 29, 1977. PA0 Semrau, Konrad, Practical Process Design of Particulate Scrubbers, CHEMICAL ENGINEERING, pp. 87-91, Sept. 26, 1977. PA0 Calvert, Seymour, Upgrading Existing Particulate Scrubbers, CHEMICAL ENGINEERING, pp. 133-140, Oct. 24, 1977. PA0 Gilbert, William, Trouble Shooting Wet Scrubbers, CHEMICAL ENGINEERING, pp. 140-144, Oct. 24, 1977.
As is set out in those publications, the ultimate efficiency of a wet scrubber is a direct function of the total energy loss in turbulence per unit volume of gas treated by the scrubber. In theory at least, any scrubbing device regardless of its design will achieve the same degree of particulate removal if it is operated at the same total turbulence per unit volume of gas. Energy to create turbulence for gas-liquid contacting can be supplied in three ways. It may be extracted from the energy of the gas stream, from the energy of the liquid stream, or from mechanical agitation of the two streams. Of course any combination of these three types of energy input may be utilized but the result is the same.
Wet scrubbers are often characterized as being either of low-energy or high-energy types. While there is no clear line of distinction between the two, a low-energy scrubber is generally characterized as having a power input in the range of about 0.75 to about 3 hp per 1000 cfm and a high-energy scrubber is considered to be one having a power input of about 3 to 6 hp per 1000 cfm. There is a practical limit as to the total energy which can be supplied for gas-liquid contacting because of the problems involved in efficiently coupling an energy source to the fluid streams.
It has long been recognized that the collection efficiency of particulate scrubbers can be increased by operating a scrubber under conditions such that condensation from water vapor from the gas stream occurs during the scrubbing process. Condensation will occur in the scrubber if the water dew point of the gas stream is above the temperature of the scrubbing liquid. Conversely, if evaporation occurs during the scrubbing process, as would be the case wherein the scrubbing water is at a higher temperature than the incoming gas stream, collection efficiency of the scrubber is decreased. Data supporting these observations are contained in a paper by K. T. Semrau and C. L. Witham entitled, "Condensation and Evaporative Effects in Particulate Scrubbing", presented at the Air Pollution Control Association, 68th Annual Meeting, June 15-20, 1975.